Thermodynamics of Adsorption of Proteins and Carboxymethyl Cellulose at Solid-Liquid Interfaces

The adsorption of proteins (bovine serum albumin and pepsin) and sodium carboxymethyl cellulose on the surface of barium sulphate particles has been measured at various ionic strengths, pH, temperatures and neutral salts. The adsorption isotherms appear to be similar to the Langmuir isotherm. In many cases, binding isotherms are found to be S-shaped at low range of solute concentration (c_p). At high solute concentrations of the macromolecules, Γ_p appears to reach a constant value Γ_p - which becomes insensitive to further increase of c_p. In the case of proteins, Γ^m_p is found to be less than 1 mg per sq metre of barium sulphate whereas for NaCMC, this value may be greater than one. Based on Langmuir equation the maximum amount of adsorbate required Γ^l_p to cover the surface of powdered barium sulphate has also been calculated, Γ^l_p is always found to be higher than Γ^m_p by twenty to forty percent depending on the ionic strength, pH and other solution parameters. The standard free energy changes (ΔG₀ per gm of the adsorbent using appropriate reference states have been calculated by the appropriate equations and it has been found that the free energy values are in the same order of magnitude of Γ^m_p so that the values correctly predict the maximum extent of adsorption. Adsorption of BSA on the mixture of barium sulphate and alumina in various proportions has shown that Γ^m_p or Γ^l_p calculated theoretically based on the addtively rule is always higher than those observed experimentally. From the variation of ΔG₀ with temperature, the respective change in enthalpy(ΔH^av₀) and entropy (ΔG^av₀) for the adsorption processes heve been calculated and the results have been critically analysed.